Astrocytes are the major glial cell type in the human brain and are critical for central nervous system (CNS) development and function. Astrocyte dysfunction has been implicated in human neurodevelopmental and psychiatric diseases. Given the important roles astrocytes play in CNS development and health, it is surprising that little is known about the molecular and cellular mechanisms that govern astrocyte maturation and their functional interactions with neighboring cells. One caveat for currently available model systems is the inability to manipulate and observe astrocyte development and dynamic changes in living, intact animals. Transparent zebrafish larvae would be ideally suited for such studies, but bone fide astrocytes have not been described in this system to date. Here, we aim to exploit the optical transparency of zebrafish larvae together with molecular and genetic approaches to characterize a new glial cell type in zebrafish that features key hallmarks of mammalian astrocytes. We have identified a previously unreported cell type in zebrafish CNS with several defining characteristics of mammalian astrocytes, such as intricate bushy morphology, glutamate transporter expression, and spontaneous microdomain Ca2+ transients. In this proposal, we will: 1) fully characterize the development and function of astrocytes in zebrafish; 2) develop a cell-specific CRISPR/Cas9 method to study astrocyte biology in vivo. We expect our work will establish zebrafish as a new model system to explore astrocyte development and function, provide new insights into the molecular and cellular mechanisms regulating astrocyte development and growth, and lay the foundation to study astrocyte function in the context of the entire nervous system in an intact and behaving animal.